This invention relates generally to television decoders and particularly to addressable cable television decoders capable of receiving data during the vertical intervals of a conventional television signal.
Most cable television systems utilize a form of signal scrambling to preclude unauthorized use of the television signals. A common form of television signal scrambling involves suppressing the synchronizing pulses and/or inverting the video information, both on a random basis. In order to receive a usable television signal, a viewer requires a decoder for restoring the suppressed synchronizing pulses and for decoding the video information. Unscrambling of the scrambled television signal is accomplished in response to appropriate decoding signals that are sent along with the scrambled signal by the cable operator.
Generally an addressable cable decoder is provided for receiving encoded data signals carried on one or more horizontal scan lines occurring during the vertical interval of a standard television signal. Typically, horizontal lines 10-13 are used. The decoder includes means for identifying the selected horizontal lines, usually by counting horizontal lines occurring during the vertical interval. Counting is initiated by a timing signal generated in response to the integrated vertical sync pulse. The decoder also includes logic means for producing signals for unscrambling the video portions of the scrambled television signal. The signals generally produced are the so-called VNX and VIX signals, representing video normal and video inverted, respectively. Application of these signals to control the operation of direct and inverting video amplifiers enables unscrambling of the video signal, which is then applied to a modulator operating at VHF channel 3/4 frequency. Since the data on the horizontal lines in the vertical blanking interval would appear as white streaks on the television screen, blanking means are provided for rendering the direct video amplifier ineffective during the trace interval until line 14. The video is never inverted during the vertical interval, and the inverting video amplifier is therefore not active. The VNX signal, which also serves other purposes in the decoder, is used to control the blanking function. The VNX signal operates to control a transmission gate in series with the direct video amplifier to remove any information between horizontal sync pulses.
Those skilled in the art will appreciate that a standard television signal includes six pre-equalizing pulses followed by a serrated vertical sync pulse and six post equalizing pulses. The pre and post equalizing pulses and the serrations in the vertical pulse all occur at twice the normal horizontal line frequency. In the prior art circuit, the VNX signal commences blanking at about horizontal line 7 with the result that some of the post equalizing pulses are eliminated from the television signal since alternate post equalizing pulses appear as data between horizontal sync pulses. Specifically, equalizing pulses corresponding to horizontal lines 7, 8 and 9 are eliminated.
While most television receivers function satisfactorily without the three missing post equalizing pulses in the television signal, some video cassette recorders need the full complement of equalizing pulses in the television signal for timing purposes. Elimination of some of the post equalizing pulses creates problems in the operation of such video cassette recorders.